Optical storage media with a super resolution near field structure (Super-RENS) offer the possibility to increase the data density of the optical recording medium by a factor of three in one dimension as compared with regular optical recording media. This is possible by a so-called super RENS structure, which is placed above a data layer of the optical recording medium, and which significantly reduces the effective size of a light spot used for reading from and/or writing to the optical storage medium. The super resolution layer corresponds also, in a simplified picture, with a mask layer, because it is placed above the data layer and only the high intensity center part of a laser beam can penetrate the Super-RENS layer. However, also other types of Super-RENS layers are known where the reflectivity in the center point of the laser beam is increased.
A super resolution near field technique for recording and retrieving small marks beyond the optical diffraction limit is described be Tominaga, Nakano and Atoda in “An approach for recording and readout beyond the diffraction limit with an Sb thin film”, Applied Physics Letters, Vol. 73, No. 15, 12 Oct. 1998, which describe to use an Sb thin film as the super resolution layer.
The Super-RENS layers at present under development for future optical storage media have the drawback that a high laser power is needed to heat the mask layer and the respective surrounding protection layers.
It is known that also semiconductor materials can be used as a mask layer for Super-RENS optical storage media, for example ZnO. A semiconductor material of this kind for a Super-RENS layer is described by Takamori et al, “Energy-Gap-Induced super-Resolution Optical Disc using ZnO Interference Film”, Japanese Journal of Applied Physics, Vol. 44, No. 5b, 2005, pp. 3627-3630. Takamori et al describe a Super-RENS disc with ZnO as an active layer deposited on a ROM type substrate and show that a temperature rise can locally increase the ZnO transmittance, thus triggering a near field interaction capable of below-diffraction-limit detection.
In the articles Hyot et al, “Phase change materials and Super-RENS”, E*PCOS 05, Technical Digest, Cambridge, 2005, and Pichon et al, “Multiphysics Simulation of Super-Resolution BD ROM Optical Disk Readout” 2006 IEEE, 0-7803-9494-1/06, PP 206-208, a semi-conducting mask layer is proposed in which a local change of the refractive index can be obtained through photo generation of free carriers. A thermal description is given to provide information on temperature distribution during readout of the data layer.
In US2003/0193857 an optical disc comprising a Super-RENS mask layer is described comprising a semiconductor film which can have a contamination or a matrix material mixed into the semiconductor which is not more than 20 at %. The Super-RENS detection is based on an increase of the transmittance of the mask layer, which the transmittance being increased by absorption saturation of the semiconductor layer upon radiation with an incident laser beam. The mask layer may include impurities, which allows to shift the energy gap such that efficient absorption is obtained for a certain wavelength. An embodiment is described for which a GaP layer can be utilized as a Super-RENS layer when doping the GaP layer with Be, which provides an acceptor level, or Te, which provides a donor level. This allows an absorption saturation of the GaP layer by providing electron excitation when using a reproduction beam having a wavelength of 650 nm. For providing absorption saturation, a comparatively high laser power is required, for example 1.3 mW when using a pulsed laser source.